Fire-retardant gypsum panels and other products of many useful shapes can be produced by dehydrating and rehydrating gypsum, also known as calcium sulfate dihydrate or landplaster. Such gypsum products are commonly used in building construction. Typical gypsum products include a dry wall and gypsum-fiber board. A dry wall comprises a gypsum core sandwiched between cover sheets such as paper or polymeric mats. A gypsum-fiber board comprises a gypsum core in which fibers are incorporated, but no cover sheets are used, although the gypsum core in a gypsum-fiber board may be coated with various coatings.
As gypsum by itself is not water-resistant, there has been a considerable amount of work on improving water resistance of gypsum products. Hydrocarbons, including wax and asphalt are suggested to reduce water uptake in U.S. Pat. No. 2,198,776. Materials including metallic soaps and silicones have been used as coatings on the surface of a finished gypsum product. However, it is difficult to obtain a coating made of these materials with consistent water resistance.
U.S. Pat. No. 4,411,701 discloses adding alkaline metal alkylsiliconates or phenylsiliconates together with calcium hydroxide or calcium oxide to gypsum in making waterproof gypsum molded products, while U.S. Pat. No. 4,371,399 discloses a water-repellent gypsum mortar comprising certain fatty amines. U.S. Pat. No. 7,294,195 discloses water repellant gypsum compositions comprising a hydroxyl-based cellulose and a siliconate.
Siloxane emulsions can be added to a gypsum slurry and these emulsions are useful for improving water-resistance of a gypsum product. A siloxane emulsion was added to gypsum-fiber boards in U.S. Pat. No. 5,817,262 to Englert. In addition to siloxane emulsions, siloxane dispersions can also be used for improving water-resistance of a gypsum product, as described in U.S. Pat. No. 7,413,603 to Miller.
Adding siloxane to a gypsum slurry improves significantly water resistance of a resulting gypsum product. For example, the use of polymethylhydrogen siloxane for producing a water-resistant gypsum product is provided by EP 1 112 986 A1.
During manufacturing, a gypsum product is subjected to a drying process and is exposed to high temperatures. During the drying process in an oven (kiln), siloxane evaporates out of the gypsum product and is carried to burners. Decomposed pieces of siloxane are burned and form very fine powdery siloxane dust. This dust reduces drying efficiency of the kiln and results in increased wear and tear of equipment including burners and fans.